The present invention relates to the field of hydrodynamic bearing spindle motors for disc drive data storage devices and, more particularly, to a motor having a lubricant with an electrically conductive, nonmetallic additive.
Disc drive data storage devices, known as "Winchester" type disc drives, are well-known in the industry. In a Winchester disc drive, digital data is written to and read from a thin layer of magnetizable material on the surface of rotating discs. Write and read operations are performed through a transducer which is carried in a slider body. The slider and transducer are sometimes collectively referred to as a head, and typically a single head is associated with each disc surface. The heads are selectively moved under the control of electronic circuitry to any one of a plurality of circular, concentric data tracks on the disc surface by an actuator device. Each slider body includes a self-acting air bearing surface. As the disc rotates, the disc drags air beneath the air bearing surface, which develops a lifting force that causes the slider to lift and fly several microinches above the disc surface.
In the current generation of disc drive products, the most commonly used type of actuator is a rotary moving coil actuator. The discs themselves are typically mounted in a "stack" on the hub structure of a brushless DC spindle motor. The rotational speed of the spindle motor is precisely controlled by motor drive circuitry which controls both the timing and the power of commutation signals directed to the stator windings of the motor. Typical spindle motor speeds have been in the range of 3600 RPM. Current technology has increased spindle motor speeds to 7200 RPM, 10,000 RPM and above.
One of the principal sources of noise in disc drive data storage devices is the spindle motor. Disc drive manufacturers have recently begun looking at replacing conventional ball or roller bearings in spindle motors with "hydro" bearings, such as hydrodynamic or hydrostatic bearings. A hydro bearing relies on a fluid film which separates the bearing surfaces and is therefore much quieter and in general has lower vibrations than conventional ball bearings. A hydrodynamic bearing is a self-pumping bearing which generates a pressure internally to maintain the fluid film separation. A hydrostatic bearing requires an external pressurized fluid source to maintain the fluid separation. Relative motion between the bearing surfaces in a hydro bearing causes a shear element which occurs entirely within the fluid film such that no contact between the bearing surfaces occurs.
In a hydro bearing, a lubricating fluid such as air or liquid, provides a bearing surface between a stationary member of the housing and a rotating member of the disc hub. In addition to air, typical lubricants include oil or ferromagnetic fluids. Hydro bearings spread the bearing surface over a large surface area in comparison with a ball bearing assembly which comprises a series of point interfaces. This is desirable because the increased bearing surface decreases wobble or run-out between the rotating and fixed members.
However, a typical hydro bearing lubricant is electrically non-conducting. During operation, electrostatic charge may build up on the disc surface. Without an electrical path from the disc surface to the disc drive housing assembly, the electrostatic charge may discharge through the read/write heads. This can cause loss of data or damage to the heads themselves, particularly in magnetoresistive (MR) heads which are very susceptible to damage by electrostatic discharge. Conductive lubricants have been used to provide an alternative conducting path. Conventional conductive lubricants, such as ferrofluids, use metallic or magnetic particles in suspension to provide electrical conductivity. These fluids usually have very high viscosity and have poor anti-wear performance.
Viscosity and anti-wear performance are important considerations in miniature hydra bearings for disc drives. The lubrication properties that must be controlled and the degree of control that must be obtained are unique to these bearings. In addition to viscosity and anti-wear, other important properties include power dissipation, migration, vapor pressure and evaporation rate, resistance to oxidation and corrosion, rheology, boundary properties and system compatibility. Viscosity determines power dissipation and bearing stiffness, which should be relatively constant over various operating conditions. Miniature disc drive applications require small power dissipation and a limited oil supply that must be adequate for a long life without escaping from the bearing. The lubricant should have low migration so the lubricant does not creep out of the bearing and into the head-disc interface. The lubricant should have a high resistance to oxidation and reactivity to provide a long life for the bearing. Rheology is the deformation and flow response to shear.
The lubricant should also be compatible with the other materials of the disc drive. For example, migration or outgassing of the lubricant should not impair the interface between the head and the disc, such as by causing an increase in the sticking friction between the head and the disc or a degradation of the head structure or operation. Formulation of fluids for appropriate hydro bearing properties therefore requires different considerations than for fluids intended as general purpose lubricants.